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The fluorescence excitation spectrum of benzene at a rotational temperature of 0.3 K has been observed using both pulsed and continuously operated supersonic free jets. Individual lines of the 1B2u(ππ) ←1A1g, (610) band at 259 nm have been clearly resolved and analyzed. Rotational and Coriolis constants as derived from these resolved spectra are in agreement with those derived from previous band contour analyses of room temperature spectra. Rotational analyses of the corresponding spectral band for the He–benzene and He2–benzene van der Waals complexes are also reported. In both complexes the helium atoms are found to lie on the C6 symmetry axis approximately 3.5 A above (and/or below) the plane of the benzene ring. Sufficient rotational cooling is achieved in these supersonic expansions so that ≳95% of all benzene molecules are relaxed to the lowest rotational state available (consistent with nonrelaxation of nuclear spins).

Absorption and dispersed fluorescence spectra are reported for a variety of 1‐phenylalkynes cooled in a supersonic free jet. The 1B2(ππ)←1A1 spectra of these compounds is characterized by a near perfect cancellation of transition strength sources for the totally symmetric vibronic bands including the origin. Two skeletal distortion modes are found to dominate the spectra, inducing strong transitions by vibronic coupling to a higher state of 1A1 symmetry. These two modes are rotated in the 1B2 state relative to their configuration in the ground state. The nature of these modes is found to be largely unaffected by changes in the length of the alkyl chain, but vibrational relaxation into the chain still appears to proceed on a subnanosecond time scale.

Absorption and fluorescence spectra have been obtained for the 1B2(ππ*)←1A1 transition of a series of para‐alkylanilines cooled in a supersonic free jet. The series consists of all p‐n‐alkylanilines through n‐butyl‐ as well as p‐isopropylaniline and aniline itself. For those members of the series having an alkyl chain length of three or greater, two distinct molecular conformations are found to be populated in the jet with roughly equal likelihood. Spectra of both the extended chain conformation [p‐n‐(t)‐alkylanilines] and the coiled chain conformation [p‐n‐(g)‐alkylanilines] show that two optically active aniline vibrations are largely unaffected by changes in the length and nature of the n‐alkyl chain. One of these vibrations (the inversion motion of the –NH2 group) would be expected to be particularly weakly coupled to the torsions and bends of p‐alkyl chain. Nonetheless, fluorescence spectra obtained when this vibration is excited in the 1B2 state show that intramolecular vibrational relaxation occurs...

The reactivity of bare carbon clusters produced by laser vaporization has been studied by a fast-flow gas-phase reactor followed by F/sub 2/ excimer laser ionization and time-of-flight mass analysis. Although most small carbon clusters (C/sub n/, with n < 40) were found to be highly reactive with such small molecules as NO, SO/sub 2/, and CO, the larger even n clusters (40 < n < 80) were found to be relatively inert. This lack of reactivity was most pronounced for C/sub 60/ (Buckminsterfullerene). Inertness of the large even C/sub n/ clusters is evidence that they have closed into edgeless, spheroidal shells composed of 12 pentagonal rings and n/2 - 10 hexagonal rings. The possible role of similar shell structures in the formation and morphology of soot is considered. 16 references, 4 figures.

We observe the resonant two‐photon ionization (R2PI) spectrum of naphthalene and benzene at and just above the first ionization potential in a supersonic molecular beam. For naphthalene, the spectrum consists of direct ionization step functions indicating the relative Franck–Condon factors for transitions into the ground state ion. The spacings observed between these steps measure the ν8 vibrational frequency in the naphthalene ion. The electric field dependence of these steps is found to be consistent with a field ionization red shift of the thresholds. In benzene we observe similar direct ionization. Superimposed on the threshold steps, however, are broad features (∼100 cm−1) due to vibrational autoionization of Rydberg states at this same energy. Implications of this data on the observation of high Rydbergs in such large organic molecules are discussed.

Single vibronic level fluorescence spectra have been measured for naphthalene cooled in a supersonic free jet. Ten spectra in all have been recorded, the excitation energies ranging from 0 to 4029 cm−1 above the 1B3u(ππ*)←1Ag origin. These results reveal extensive intramolecular vibrational redistribution (IVR) occurring in the absence of collision on a time scale much faster than fluorescence even at vibrational energies as low as 2570 cm−1. Comparison of the observed spectra with model calculations shows that this IVR process involves most of the harmonic overtone/combination basis states of overall b1g symmetry having energies within the absorption line profile.

The 1B2(ππ*)←1A1 absorptions of two monosubstituted halobenzenes have been investigated using resonance enhanced two‐photon ionization in a pulsed supersonic molecular beam. Detection of the photoions was accomplished by means of a time‐of‐flight mass spectrometer. The 1B2←1A1 system of bromobenzene has been observed with good sensitivity using this technique, even though the total decay rate of the 1B2 state is greater than 1×1011 sec−1. No ion signal was observed when the same transition was probed in iodobenzene, allowing us to place a lower limit on its decay rate of 4×1013 sec−1.

The lifetime of triplet benzene produced by laser excitation of the 6/sup 1/ level of the S/sub 1/ state in a supersonic beam has been measured by photoionization with an ArF excimer laser. This collision-free lifetime pertaining to the vibrationally unrelaxed triplet was found to be 470 +- 50 ns. Measurement of a component of the photoion signal due to decay of S/sub 1/ permitted, in addition, an estimate of the quantum yield for intersystem crossing.

The dissociative chemisorption of H2 on supersonic beams of Nb, Co, and Cu clusters is studied. (AIP)